your comment about the swarf, id considered. I cant remember if the down tube is open at the bottom bracket ... pretty sure it isnt on this cinder cone. ill pop the bottom bracket out to check.Centre punch it and drill carefully and slowly with a sharp HSS drill bit. Start with a small drill and work your way up to 4mm or a touch under if youre using imperial sized drills. Then run the tap through and that wlll push any remainung bits of the old bolt into the frame so make sure youve got access into the down tube to get the swarf out